1. Field of the Invention
This invention relates to flexible conductive foil printed circuit patterns which are commonly called lead frames and may be laminated to flexible dielectric strip carriers. More particularly, this invention is concerned with flexible sprocketed lead frame tapes having a plurality of printed circuit patterns arranged sequentially thereon which are provided with a plurality of fragile delicate lead fingers which are referred to as inner leads in this industry and are adapted to be connected to terminal pads on a semiconductor device.
The novel improved printed circuit lead carrier tape of the present invention is adapted to be supplied in reel form to be used on bonding machines and assembly machines for automatic and semiautomatic production of electronic modules or devices.
2. Description of the Prior Art
Very early semiconductor chips were connected to the post of metal headers which constituted the lead out terminals for the semiconductor device by means of fine wire consisting of gold and aluminum. Originally such wire bonded connections were made manually employing special wire bonding machines. Due to labor costs these manual operations have been semiautomated and fully automated at the present time.
Numerous attempts have been made to fabricate the equivalent of a fine wire by making a printed circuit pattern on a rigid substrate and providing the semiconductor chip with bumps at the terminals or pads. The chip was then placed face down on the pattern in proper registration and all of the bumps connected to the pattern at the same time. This technique or method has been commonly referred to as flip chip bonding.
The printed circuit or foil pattern has also been placed on a flexible dielectric carrier or substrate in a form which provides the equivalent of wire bonded leads in the printed circuit pattern. Byrne et al. U.S. Pat. NO. 3,544,857 recognized that the printed circuit pattern should be stabilized and provided a throw away carrier which was peeled off after the pattern was bonded to the semiconductor chip. The fingers or leads in this device required that the bonding take place by applying the bonding force through the compliant peel away carrier. The outer ends of the leads of the Byrne et al device were connected to printed circuit patterns which constituted a portion of the outer leads.
Hugle U.S. Pat. No. 3,440,027 mounted a foil pattern on an indexed insulative carrier substrate and provided printed circuit leads which were the equivalent of wire interconnections which could be separated from the carrier after the leads were bonded to a chip in a manner similar to that anticipated by Byrne et al.
Aird U.S. Pat. No. 3,689,991 refined the Hugle and Byrne et al. techniques employing foil patterns and carrier dielectrics to provide a plurality of true cantilevered beams which extended out over a device aperture to provide inner lead fingers. The dielectric carrier tape also became a support for the outer end of the printed circuit pattern leads which provided outer cantilevered beam leads. This structure permitted the free ends of the conductive foil pattern to be bonded directly to terminal pads of the semiconductor chip and then to the lead out terminals on a substrate as a substitute for wires.
The semiconductor industry has progressed by providing higher density integrated circuit devices at lower cost. Progress is directed to increasing the number of devices on a given area and to increasing the speed of operation of the devices on the integrated circuit chip. By shortening the distances between the discrete elements on the integrated circuit, the time of propagation is decreased. Further, the intercapacitive and interconductive values for the discrete elements are reduced as the elements are made smaller which permits increased frequency of operation of the device.
As integrated circuit devices are made smaller, the terminal pads or lead out pads on the integrated circuits are also being made smaller. Due to the face that it is difficult or near impossible to wire bond small wires on a pad which is less than four mils in diameter or rectangular shape, there is a limit to the ability to automatically wire bond high density very large integrated circuit devices.
In the present state of the art, a 1.3 mil gold wire can be successfully bonded in an automatic wire bonder to a pad which has a rectangular size of approximately four mils separated on four mil centers. As these pads and separation distances are made smaller, the effectiveness of wire bonding is appreciably dimenished.
It is possible to make flexible lead carrier tape having a photo lithographically produced printed circuit pattern of conductive flexible foil thereon which is very accurate. For example, the cantilevered fingers may be produced which are only two mils wide and are capable of being bonded to two mil rectangular or circular terminal pads on four mil centers. Such accurate leads may be connected to the terminal pads of a semiconductor device using my Tape Automatic Bonding (TAB) machine which is described in my U.S. Pat. No. 4,050,618. It has been shown that as many as 240 such cantilevered leads can be bonded simultaneously to a semiconductor chip employing accurately controlled conditions. When TAB bonding 240 or more two mil leads to an integrated circuit device it was found that handling the fragile delicate fingers required special handling techniques to avoid bending any one of the leads which would render the circuit pattern inoperative for commercial bonding. Commercial lead carrier tapes are presently made with four mil wide fingers and as many as fifty percent of the tape sites or circuit patterns which reach the bonding station have at least one or more of the fragile delicate fingers bent which requires rejection rework and/or manual straightening of the fingers. The alternatives to straightening the fingers before reaching the bonding station is to reject the partially completed device and/or manually repairing the circuit pattern an a repair station. The problem of bent cantilevered lead fingers has existed as long as this technique has been used in the industry. As the number of cantilevered finger leads increases on each circuit pattern tape site and the width of the lead finger decreases toward the two mil or less dimension, the rate of damaged tape sites reaching the bonding station will increase exponentially. The problem has long been recognized as reducing yields in assembling devices. The aforementioned Aird Patent attempted to supply a dielectric carrier support intermediate the ends of the leads. Other manufacturers have attempted to screen on or fabricate non-removable dielectric rings, intermediate the ends of the cantilever beams to effectively shorten the length of the cantilevered beams and increase stability of the lead or finger. However, these attempts to stabilize the fragile delicate cantilevered lead fingers have proven unsuccessful and the problem of such leads being bent in the vertical or lateral direction remains unsolved.
It would be desirable to provide a conductive foil printed circuit pattern on a flexible conductive tape or a flexible foil printed circuit pattern on a dielectric carrier tape with cantilevered lead fingers of approximately two mils width so as to accomodate the presently anticipated highest density VLSIC chips being manufactured in the laboratories today. Further, it would be desirable to increase the yields of lead carrier tape manufacturers and the yields on TAB bonding machines when employing high density fragile delicate lead fingers on the circuit patterns being attached to the integrated circuit chips.